1. Field of the Invention
The invention relates to a pavement marker that reflects and/or transmits light for identifying selected regions of a driving surface.
2. Description of the Related Art
Pavement markers are secured on or in a pavement surface to help direct drivers along preferred travel lanes. A typical pavement marker includes a plurality of separate components selected to perform specified functions. For example, a typical pavement marker may include a base and at least one light transmissive or retroreflective panel mounted to the base to produce optical signals that will be visually apparent to a driver approaching the pavement marker. The base of a surface-mounted pavement marker typically may be molded from a thermoplastic material and may include a bottom surface configured for affixation on the surface of the pavement. The base may further include a top area configured for receiving the light transmissive and/or retroreflective panel.
Injection molded plastic products that are thick require a long time to cure and create the potential for sink marks or other dimensional irregularities. As a result, most injection molded products are formed with an array of coring holes separated by a unitary matrix of thermoplastic webs. The webs are designed to be of substantially uniform thickness. The thickness and arrangement of the webs is selected to ensure adequate strength for the thermoplastic product and to achieve substantially uniform curing of the thermoplastic material. The base of a surface-mounted pavement marker typically is designed with coring holes for the above-described reasons.
Coring holes that extend in a side-to-side or front-to-rear direction on the base of a pavement marker could affect the strength of the base and could lead to failure in response to impact by vehicular tires. Coring holes that extend in a top to bottom direction are much less likely to create stress related failures. However, coring holes that extend downwardly into the top surface of the base require a separate cover to prevent accumulation of liquid or debris. The coring holes can be covered by the lens or retroreflective panel of the pavement marker.
The assignee of the subject invention has determined that a large lower surface area helps to retain the pavement marker in a fixed position on the pavement. Coring holes that extend into the bottom surface of a pavement marker necessarily reduce the surface area, and hence can adversely affect the retention of the pavement marker on the pavement. Copending U.S. patent application Ser. No. 10/442,336 discloses a pavement marker with a bottom cover that closes the coring holes and increases the bottom surface area.
The above-described components of a pavement marker must be securely affixed to one another. Ultrasonic welding offers many manufacturing advantages and efficiencies for such affixation. For example, ultrasonic welding avoids difficulties of selecting an appropriate adhesive that is compatible with both of the materials being adhered. Additionally, ultrasonic welding avoids the additional manufacturing steps of applying adhesive, as well as environmental, health and safety concerns relating to the use of some adhesives.
The typical ultrasonic welding apparatus employs an anvil for supporting one of the objects that are to be welded and a horn for disposition against the other of the objects to be welded. The horn applies high frequency sound energy near a selected weld location. The sound energy excites the molecules in the plastic and causes the plastic to melt near the application of the sound energy. The plastic will harden shortly after the ultrasonic energy has been stopped and the two materials will weld together.
Ultrasonic energy may be used in the above-described pavement marker by applying the ultrasonic energy to locations on the panel or cover opposed to the webs that separate the coring holes. The ultrasonic energy is intended to melt or soften the thermoplastic material of the panel or cover so that the melted thermoplastic of the panel or cover fuses with the opposed thermoplastic material of the web.
Molten material produced by ultrasonic welding tends to spread transversely away from the center of the application of ultrasonic energy. Thus, the molten plastic will flow away from the webs that support the panel or cover on the pavement marker. These wide pools of molten thermoplastic material then harden. However, a substantial portion of the hardened pool of plastic will contribute little or nothing to the secure attachment of the panel or cover to the base. Consequently, the energy expended and the time allotted for ultrasonic welding is not always commensurate with the benefit achieved. Additionally, the strength of some ultrasonic welds may not be adequate to withstand stresses imposed by vehicular tires.
Some pavement markers include a fluorescent top panel designed to produce an edge glow. More particularly, fluoresced photons are emitted within the marker top and strike the bottom and/or top surfaces of the top panel. The fluoresced photons that strike these top and bottom surfaces at appropriate angles will be reflected internally and redirected within the panel. A significant portion of these internally reflected photons will be emitted from the edge of the panel to produce an edge glow that will be visually apparent to a driver. Pavement markers desirably are designed to maximize the edge glow. Any geometry of the panel that deviates from a smooth flat surface will allow photons to escape, and hence will decrease the effect of the edge glow. The above-described ultrasonic welding pattern effectively produces a wide frozen puddle of plastic, and hence creates a substantial deviation from the initially planar surface of the plastic. Accordingly, ultrasonic welding can reduce the edge glow effect.